1. Technical Field
The invention relates to an apparatus and method for predetermining the operating pressure of an enhanced oil recovery gas flood and, more particularly, for determining the minimum miscibility pressure of a gas in a liquid.
2. Description of Related Art
The minimum miscibility pressure of a gas in a crude oil is a critical parameter in the operation of gas floods employed as enhanced oil recovery processes. The minimum pressure at which a gas is miscible in the crude oil in place is the basis for optimizing the operating pressure of the gas flood. It is generally preferable to conduct the flood at or slightly above the minimum miscibility pressure (MMP) of the flooding gas in the crude oil because considerably more oil is recovered when the gas is miscible in the oil than when it is immiscible. Furthermore, increasing the pressure substantially above the MMP does not significantly improve oil recovery while exposing the operator to additional operating costs, increased risk of undesirable formation fracturing, and increased safety hazard.
A number of methods and apparatus are known for determining the MMP of a gas in an oil, including the widely accepted slim tube method. A slim tube is a long narrow tube approximately 12.2 to 18.3 meters long and having an inside diameter of 0.64 cm or less and packed with an unconsolidated material such as sand or glass beads. The tube is saturated with oil and thereafter flooded with a gas at constant pressure and temperature. The oil recovery is determined at that pressure and then similar floods are conducted at different pressures. The oil recovery at each pressure is measured as a function of the volume of gas injected. The oil recovery efficiency is determined thereafter as a function of flooding pressure. The MMP, as determined by the slim tube method, is the pressure above which there is very little increase in oil recovery efficiency. The slim tube method is extremely time-consuming, taking several days to determine the MMP of a single gas-crude oil system. See Yellig and Metcalf, "Determination and Prediction of CO.sub.2 Minimum Miscibility Pressures," J. Pet. Tech., v. 32 (1980) pp. 160-168.
U.S. Pat. No. 4,455,860 to Cullick et al describes a method for determining the MMP of CO.sub.2 which comprises filling a capillary tube with oil and injecting CO.sub.2 into the tube at a fixed temperature and pressure to displace the oil. The MMP is calculated from pressure response data.
Direct visual methods have been proposed for determining the MMP of CO.sub.2 in crude oil. In Wang and Knight, "Visual Study of Miscibility Development of CO.sub.2 -Crude Systems," Proceedings of the European Symposium at Paris, Nov. 8-10, 1982, pp. 269-278, a CO.sub.2 -rich vapor phase and a crude oil-rich liquid phase are maintained in a visual cell at a constant temperature and pressure until equilibrium is attained. The pressure is gradually increased and the CO.sub.2 is recirculated through the oil, adding additional CO.sub.2 as needed, to maintain the pressure until a third CO.sub.2 -rich liquid phase develops between the first two phases. The pressure is further increased until the interface disappears between the CO.sub.2 -rich liquid phase and the CO.sub.2 vapor phase. This is defined therein as the MMP. A second test is performed to confirm a multiple contact miscibility mechanism. In the second test, a CO.sub.2 -rich liquid phase and a crude oil liquid phase are present in the cell. Oil droplets are released into the CO.sub.2 -rich liquid phase at the top of the visual cell. The droplets gradually enrich the CO.sub.2 -rich liquid phase until the droplets become miscible therein, confirming the multiple contact miscibility mechanism.
Although the direct visual determination of MMP appears to be more straightforward than other known experimental methods, a more rapid and reliable visual method is needed which more closely approximates the multiple contact miscibility mechanism of a miscible gas flood in a formation by a single experimental procedure.